Recently, light emitting devices have been considered for use as display devices. The light emitting device comprises an electron injection electrode, a hole injection electrode and an emission unit formed between the electron injection electrode and the hole injection electrode. When injecting electron charges into the emission unit, the injected electron and the injected hole are paired, and then the extinction of the injected hole-electron pair results in electroluminescence. There has been extensive research focused on developing a light emitting device with a low driving voltage, a wide viewing angle, a decrease in power consumption, an excellent characteristic in light-weight, a color, and the like. However, because the light emitting device has a short operational life span, extensive commercialization of the light emitting device has not yet been achieved.
The life span of the light emitting device is determined by two factors. One factor is a decrease in luminance of the light emitting device when driving the light emitting device. The decrease in luminance of the light emitting device is caused by impurities in an organic material forming an emission layer, an interface between the organic material and the electrode, a low glass transition temperature (Tg) of the organic material, oxidation of the light emitting device by oxygen and moisture. The other factor is that even if the light emitting device is not in use, an emission region gradually decreases by moisture and therefore the light emitting device fails to emit light. Thus, the life span of the light emitting device is determined by one factor with a smaller value of the two factors.
In a manufacturing process of the light emitting device, moisture exists inside the light emitting device. Moisture also permeates from the outside of the light emitting device. The moisture permeated from the outside of the light emitting device negatively effects the performance of the light emitting device. Thus, various methods had been proposed to solve the above problem relating to the moisture. The representative method is to cover the light emitting device with a shield cap made of a metal material or a glass material. This is called an encapsulation method.